FIG. 1 illustrates a typical prior art color, transmissive LCD. The structure of FIG. 1 will be used to identify certain disadvantages of prior art LCDs that are avoided by the present invention.
In FIG. 1, an LCD 10 includes a white light source 12 to provide backlighting for the upper LCD layers. A common source for white light is a fluorescent bulb. Another white light source is a combination of red, green, and blue light emitting diodes (LEDs) whose combined light forms white light. Other white light sources are known. These white light sources must provide substantially homogeneous light to the back surface of the display.
A polarizing filter 14 linearly polarizes the white light. The polarized white light is then transmitted to a transparent thin film transistor (TFT) array 16 having one transistor for each red, green, and blue subpixel in the display. An adjacent set of red, green, and blue subpixels is referred to as a white pixel whose color “dot” is a combination of the three subpixels. If the RGB subpixels are all energized, the dot creates white light. TFT arrays are well known and need not be further described.
Above the TFT array 16 is a liquid crystal layer 20, and above liquid crystal layer 20 is a transparent conductive layer 22 connected to ground. An electrical field across a subpixel area of the liquid crystal layer 20 causes light passing through that subpixel area to have its polarization rotated orthogonal to the incoming polarization. The absence of an electrical field across a subpixel area of the liquid crystal layer 20 causes the liquid crystals to align and not affect the polarization of light. Selectively energizing the transistors controls the local electric fields across the liquid crystal layer 20. Each portion of the liquid crystal layer associated with a subpixel is commonly referred to as a shutter, since each shutter is controllable to pass from 0-100% (assuming a lossless system) of the incoming light to the output of the display. Liquid crystal layers are well known and commercially available.
A polarizing filter 24 only passes polarized light orthogonal to the light output from the polarizing filter 14. Therefore, the polarizing filter 24 only passes light that has been polarized by an energized subpixel area in the liquid crystal layer 20 and absorbs all light that passes through non-energized portions of the liquid crystal layer 20. The magnitudes of the electric fields across the liquid crystal layer 20 control the brightness of the individual R, G, and B components to create any color for each pixel in the displayed image.
Other types of LCDs pass light through only the non-energized pixels. Other LCDs use different orientation polarizers. Some types of LCDs substitute a passive conductor grid for the TFT array 16, where energizing a particular row conductor and column conductor energizes a pixel area of the liquid crystal layer at the cross-point.
The light passing through the polarizing filter 24 is then filtered by an RGB pixel filter 25. The RGB pixel filter 25 can be located at other positions in the stack, such as anywhere below or above the liquid crystal layer 20. The RGB pixel filter 25 may be comprised of a red filter layer, a green filter layer, and a blue filter layer. The layers may be deposited as thin films. As an example, the red filter layer contains an array of red light filter areas defining the red subpixel areas of the display. Similarly, the green and blue filter layers only allow green or blue light to pass in the areas of the green and blue subpixels. Accordingly, the RGB pixel filter 25 provides a filter for each R, G, and B subpixel in the display.
The RGB pixel filter 25 inherently filters out at least two-thirds of all light reaching it, since each filter subpixel area only allows one of the three primary colors to pass. This is a significant factor in the generally poor efficiency of the prior art LCDs. The overall transmissivity of the LCD layers above the white light source 12 is on the order of 4-10%.
It is known to eliminate the RGB filter by sequentially energizing red, green, and blue LEDs in the backlight where the sequencing is synchronized with the control of the red, green, and blue subpixel areas of the liquid crystal layer. In this way, red, green, and blue images are rapidly displayed in sequence to create the appearance of a full color image. However, the current state of the art liquid crystal layers cannot be switched fast enough to avoid flickering and other artifacts, especially if the LCD is to be a television screen.
It is also known to have red, green, blue, and white subpixels in a single white pixel, where the white subpixel does not have any filter. Having a separate white subpixel, whose grayscale is controlled by the liquid crystal shutter, can efficiently be used to control the displayed color saturation. However, having an extra subpixel (i.e., 4 vs. 3) for a single white pixel reduces the resolution of the displayed image and reduces the color-generating areas of the RGB pixels. Further, the liquid crystal layer needs additional drive circuits, resulting in an expensive, customized device.
What is needed is a technique for increasing the efficiency of an LCD without the drawbacks of the prior art techniques.